The invention discloses a control method of surface waviness of a large-size optical element processed through a single point diamond milling method and belongs to the field of ultraprecision machining of a large-size brittle optical element. The invention aims to solve the problem that the existing SPDT (single point diamond turning) method cannot guarantee waviness errors and frequency domain evaluation index PSD1 when a large-size optical element is processed. The control method disclosed by the invention comprises the following steps: 1. carrying out rough machining; 2. acquiring a space cycle T of longitudinal strips; 3. judging rigidity of the processing lathe, executing the step 4 if the rigidity is too low and executing the step 5 if the rigidity is too high; 4. increasing average pressure between a beam and a leveling pad body and then executing step 6; 5. reducing average pressure between the beam and the leveling pad body or contact rigidity, and then executing step 6; 6. carrying out secondary ultraprecision machining; 7. detecting the PSD1 value again; 8. judging whether the fact that PSD1 is less than or equal to 15nm<2>.mm is right or false; returning to the step 2 if the fact is false; and if the fact is right, completing the control of the waviness errors and the frequency domain index PSD1 of the large-size optical element U processed through the single point diamond milling method.